1. Field of the Invention
The present invention relates generally to a broadcasting system, and in particular, to a play-out apparatus and method for switching a channel in a Digital Multimedia Broadcasting (DMB) receiver.
2. Description of the Related Art
Conventionally, digital broadcasting is a system for providing users with high-quality images, Compact Disk (CD)-quality sounds, and high-level services that can replace the conventional analog broadcasting system. The digital broadcasting system provides broadcast traffic compressed at a high compression rate according to Moving Picture Experts Group-2 (MPEG-2) and MPEG-4 standards and so on. The digital broadcasting system employs the high compression rate because of the large amount of data/information to be transmitted from the digital broadcasting system. A typical example of a digital broadcasting service is a Digital Multimedia Broadcasting (DMB) service.
The DMB service can provide various multimedia signals such as audio, video, etc. For example, as a concept of wireless broadcasting in which a broadcast can be only listened to is extended to a new concept of wireless broadcasting in which a broadcast can be listened to and viewed, various multimedia information such as traffic and news information as well as a music broadcast can be provided in the form of text, graphic, video, and so on. In case of video broadcasting, a system interworks with an existing digital broadcasting network for terrestrial broadcasting, satellite broadcasting, cable television (TV), etc., to provide various multimedia services, and interworks with a Intelligent Transport System (ITS), Global Positioning System (GPS), and so on to provide a telematics service.
Specifically, the DMB service provides high-quality sound and video broadcasts in CD and Digital Video Disc (DVD) levels to mobile terminals such as a mobile phone, Personal Digital Assistant (PDA), and in-vehicle terminal on the move as well as fixed terminals. Thus, it is predicted that there will be a significant increase use of the DMB service. The DMB service can be divided into a terrestrial DMB service and a satellite DMB service. Herein, the terrestrial DMB service can provide a broadcasting service using a terrestrial relay, and the satellite DMB service can provide a broadcasting service using a terrestrial or satellite relay.
Next, a DMB system for providing the DMB service will be briefly described with reference to FIG. 1. FIG. 1 illustrates a structure of a conventional DMB receiver.
Referring to FIG. 1, a receiver 100 receives a signal from a gap filler corresponding to a DMB satellite or terrestrial relay. The receiver 100 frequency-converts a DMB channel signal and then outputs the frequency-converted signal to a Code Division Multiplexing (CDM) demodulator 110. The CDM demodulator 110 spreads and demodulates a received broadcast channel signal using a Walsh code of an associated received channel. Transmitted packets output from the CDM demodulator 110 correspond to a Transport Stream (TS) of a selected channel. The TS is input to a Forward Error Correction (FEC) unit 120. At this time, the output of the CDM demodulator 110 is input to the FEC unit 120 on the basis of each Walsh code of the received channel. The FEC unit 120 corrects error due to noise or interference during transmission. The FEC unit 120 is provided with a bit deinterleaver 121, a convolutional code decoder 122, a byte deinterleaver 123, and a Reed-Solomon (RS) decoder 124. The bit deinterleaver 121 deinterleaves broadcast signals received from the CDM demodulator 110 in a bit unit in order to change burst error into distributed error. The deinterleaved satellite broadcast signals are input to the convolutional code decoder 122. Because the output signals of the bit deinterleaver 121 are convolutionally coded signals, the convolutional code decoder 122 inputs the signals to the byte deinterleaver 123 after error correction. The byte deinterleaver 123 deinterleaves the broadcast signals output from the convolutional code decoder 122 in a byte unit. The byte deinterleaver 123 changes burst error into distributed error in the byte unit. When the convolutional code decoder 122 does not suitably correct the error, the byte deinterleaver 123 corrects a burst error if one has occurred.
The broadcast signals output from the byte deinterleaver 123 are input to the Reed-Solomon decoder 124. The Reed-Solomon decoder 124 recovers the deinterleaved signals from error using parity data and then outputs the recovered signals to a demultiplexer (DEMUX) 130. The demultiplexer 130 separates a decoded TS into voice, video, and text data packets. The voice, video, and text data packets output from the demultiplexer 130 are input to an audio decoder 160, a video decoder 150, and a text decoder 140, respectively. The text decoder 140 decodes the compressed text data packet, such that a display unit 170 displays text data. The video decoder 150 converts a compressed video data packet into a format suitable for the display unit 170 through a video processor (not illustrated), such that the display unit 170 displays video data. The audio decoder 160 decodes a compressed voice data packet, and converts the decoded voice data packet into an analog signal through a Digital to Analog Converter (DAC) (not illustrated), such that a speaker 180 outputs voice data. A controller 190 controls the components of the DMB receiver such that a DMB service can be received. A memory 191 stores a control program for performing various control functions and data necessary for control. A key input unit 192 allows a user to change or select a service by manipulating a key for a service change request.
FIG. 2 illustrates a process for buffering video data output from a demultiplexer in a playback buffer to switch a channel in the conventional DMB receiver. An operation for switching a channel in the DMB receiver in case of the conventional satellite DMB will be described with reference to FIG. 2.
As illustrated in FIG. 2, the demultiplexer 210 receives a TS, separates the TS into video and audio data, and outputs the video and audio data to the playback buffer 220. In the present invention, the audio data is not considered under assumption that only the video data is output to the playback buffer 220.
The video data is sequentially accumulated and then is output after a predetermined time. Herein, a time stamp of the video data transmitted from a broadcasting station indicates the predetermined time. That is, when the video data is transmitted from the broadcasting station, the time stamp is included and transmitted in the video data.
The video decoder 230 decodes the video data and then outputs the decoded video data to the display unit 270, such that the display unit 270 can play the decoded video data.
In case of the conventional satellite DMB, a delay for new information acquisition occurs when a broadcasting service or channel is switched in the DMB receiver. This delay occurs for two reasons.
First, if CDM is applied to the satellite DMB receiver, a delay occurs in a process for emptying an interleave buffer and filling a deinterleave buffer when a channel is switched. That is, the delay occurs in a network used to change a CDM channel signal currently being received into a baseband signal. The network delay is defined as the delay between the receiver 100 and the demultiplexer 130 in FIG. 1.
Second, a Coder-Decoder (CODEC) delay occurs when an actual decoding task is started in a multimedia CODEC for a stable play-out process after a buffering operation of the playback buffer during a period of about 2 sec. The CODEC delay is defined as the delay until a play-out operation is performed in the video decoder 150 after the demultiplexer 130 in FIG. 1. According to the actual testing, a delay time is about 5˜6 sec when a channel is switched in the satellite DMB receiver. In this case, a network delay time is about 3˜4 sec and a CODEC delay time is about 2 sec. Thus, the delay should decrease such that the user can smoothly receive a broadcasting service without being inconvenienced when a channel is switched.